The long-term goal is to elucidate gender differences in the expression and spatial distribution of cardiac ionic channels and their functional consequences on the electrical and intracellular free Ca2+ (Cai) handling properties of the heart. Clinical studies have shown that women have longer corrected QT intervals (QTc), a greater propensity to drug-induced long QT syndrome (LQTS) and a higher incidence of sudden cardiac death than men. Differences in APD diminish after menopause implicating a modulation of ion channels by estrogen and/or testosterone. The effect of estrogen-replacement in post- menopausal women on arrhythmia vulnerability remains unknown. Female rabbits have longer APDs and QT intervals than males and a greater vulnerability to LQT-related arrhythmias. Differences in the expression and spatial distribution of IKr, IKs and/or ICa most likely account for differences in repolarization sequence, QT prolongation and enhanced susceptibility of females to arrhythmias. To characterize gender differences in Cai handling and dispersion of repolarization (DR), we propose to apply imaging techniques at high spatial and temporal resolution to simultaneously map voltage and intracellular Ca2+ transients from perfused rabbit hearts. Optical maps of APs and Cai transients q selective blockers for IKr, IKs and ICa will elucidate gender differences in the spatial distribution of these channels and will be correlated with measurements of Ito, IKr, IKs and ICa, by voltage clamping of ventricular myocytes isolated from various regions of the heart. These data will also be correlated with distributions of mRNA and protein using selective probes and antibodies for individual ion channel subunits. The specific aims are: 1) To characterize gender differences in the spatial distribution of Ito, IKr, IKs, and ICa and their functional effects on APD, repolarization and Cai handling that can be attributed to gonadal hormones. Hearts from female (q ovariectomy (OVX)), and male (q orchiectomy (ORX)) rabbits will be optically mapped (approximately 1 month later) to obtain baseline values for electrical and Cai handling. Hearts will then be treated with an IKr, IKs, and/or ICa blockers to measure changes in APD, repolarization patterns, and susceptibility to arrhythmias. Optical maps will be correlated with the distribution of ionic currents, channel mRNA levels and channel protein distribution using voltage clamp, Northern and Western blot techniques. 2) To investigate the effects of hormone replacement on the dispersion of repolarization and arrhythmia susceptibility by treating OVX and ORX rabbits with 17-beta estradiol (EST), 5alpha-dihydroxy-testosterone (DHT) for 2-3 weeks. Hearts will be examined for altered electrical and Cai handling properties, which will be correlated with changes in the distribution of ion channel currents, mRNA and protein levels. 3) To study gender differences in response to ischemic injury by determining the effects of acute and chronic ischemia (rabbit infarct model) on dispersion of repolarization and arrhythmias on males (q ORX) compared to females (q OVX). 4) To determine possible protective effects of estrogen replacement on ischemic injury. Changes in APD, dispersion of repolarization and arrhythmia susceptibility will be measured in an acute and chronic infarct model using rabbit hearts from OVX females with or without estrogen replacement. Such an investigation will analyze the functional effects of cardiac repolarization, a parameter that has been implicated in an underlying mechanism for the initiation and maintenance of arrhythmias and thus, will address a fundamental problem in women's health.